Le jour d'avant/d'après confusion

Le jour d'avant/d'après confusion

What exactly is meant by "these can only be used on their own" when talking about "le jour d'avant/d'après"? Maybe I'm missing something obvious but for me the examples don't really encapsulate the concept. Do you mean that they can't be used to detail more about the thing that happened - only that which happened before and after it? For example you couldn't say:"le jour d'avant de la visite de ma tante" (the aunts visit being the springboard for what has happened the day before or after)

Actually, I think my example "de la visite de ma tante" was not right in this context as it doesn't involve a subject at the beginning? In any case, hopefully the rest of my question/examples still stand.

Le jour d'avant/d'après confusion

What exactly is meant by "these can only be used on their own" when talking about "le jour d'avant/d'après"? Maybe I'm missing something obvious but for me the examples don't really encapsulate the concept. Do you mean that they can't be used to detail more about the thing that happened - only that which happened before and after it? For example you couldn't say:"le jour d'avant de la visite de ma tante" (the aunts visit being the springboard for what has happened the day before or after)